1. Field of the Invention
This invention relates to a photosensitive composition having high sensitivity at a near infrared ray region, particularly to a positive type photosensitive composition which is possible to directly make a plate by using a near infrared ray laser with digital signals such as a computer and the like.
2. Background art
A positive type printing plate generally has a form in which a photosensitive coating layer is provided on a support by coating. It is a printing plate of a type that when the photosensitive coating film is exposed to a suitable radiation, a solubility to a developer of the photosensitive coating film at the exposed portion is increased than that of the unexposed portion whereby an image is formed and a plate is produced. The image region (unexposed portion to irradiation) remaining after plate-making is ink-receptive or hydrophobic and the region (exposed portion to irradiation) at which the film is dissolved and removed is water-receptive or hydrophilic by using a hydrophilic support or providing a hydrophilic layer between the photosensitive coating film and the support. In the field of the positive type lithographic PS plate, as a photosensitive coating film having such a function, it has been proposed a technique in which o-naphthoquinonediazide-5-sulfonic acid derivative and a phenol resin as disclosed in, for example, Japanese Patent Publications No. 3627/1962, No. 1954/1962, No. 28406/1968 and No. 9610/1970. However, irradiations disclosed in these references are ultraviolet rays corresponding to the absorption wavelength at the ultraviolet region possessed by the o-naphthoquinonediazide-5-sulfonic acid derivatives. It is obvious that the films disclosed in the above-mentioned references do not have any light-sensitivity to the irradiation at the near infrared region.
On the other hand, it is remarkable in progress of laser beam in recent years, and particularly, a high power and small sized semiconductor laser which irradiates infrared rays at the wavelength of 760 nm to 1200 nm or a solid laser can be easily available. By using these lasers as a recording light source, it becomes possible to directly make a plate with a high resolution from digital data of a computer, or the like. Thus, research and development of a heat mode plate-making material by a laser beam have become more aggressive. For example, in Japanese Registered Patent No. 2,577,718, a technique of using a phenol resin, a heat-decomposable onium salt and a near infrared rays-absorbing dye in combination is disclosed. Also, in Japanese Provisional Patent Publications No. 20629/1995, No. 138500/1997, No. 185160/1997 and No. 211863/1997, a technique of using a phenol resin, a latent type Bronsted acid and a near infrared rays-absorbing dye in combination is disclosed. By suitably controlling kinds, molecular weights, composition ratio of the materials disclosed in the above references, a positive type or a negative type infrared rays-sensitive printing plate can be obtained. However, it is the present status that an infrared rays-sensitive printing plate having both of high infrared rays-sensitivity, and excellent storage stability with a lapse of time as well as high printing endurance simultaneously has not yet been developed.
Also, in Japanese Patent Publication No. 44416/1981, a technique in which an alkali-soluble resin, a basic compound and a photo-oxidation agent are used in combination is disclosed. On the other hand, in Japanese Provisional Patent Publication No. 60733/1974 or Journal of Imaging Science, vol. 34, p. 50 (1990), a technique of using a basic compound having a nitro group as a substituent which functions as a photooxidation agent in the molecule and an alkali-soluble resin in combination is disclosed. However, the irradiation to be used for recording in these techniques is ultraviolet rays corresponding to absorption at the ultraviolet region possessed by the photo-oxidation agent itself or the basic substance itself. Thus, it has never been known about a combination of recording materials having sensitivity to infrared rays.
Moreover, in Japanese Provisional Patent Publications No. 3165/1998 and No. 153863/1998, a positive type photosensitive composition containing a resin having a hydrophilic group, a thiopyrylium salt having a specific structure or a metal chelate compound or a boron salt, and an infrared rays-absorbing dye in combination is disclosed. In these techniques, by mixing three components of (1) a resin, (2) an infrared rays-absorbing dye and (3) a thiopyrylium salt having a specific structure or a metal chelate compound or a boron salt, aggregates hardly soluble in an alkaline treating solution are formed and decomposition of the aggregates is induced by on/off of heat generated by an infrared laser irradiation, and the laser irradiated portion is dissolved and removed by the alkaline treating solution to form an image. Also, in Japanese Provisional Patent Publication No. 43847/1997, there is disclosed a technique in which a thin film is formed by a mixture comprising two components of a novolak resin and an infrared rays-absorber, change in crystallinity of the thin film is induced by irradiating infrared laser beam, whereby a resist material is formed by utilizing change in solubility of the thin film in the alkaline treating solution. However, laser sensitivities in these techniques are low and difference in solubilities between the infrared laser-irradiated portion and unirradiated portion to the alkaline treatment solution is small so that it is difficult to form a clear and sharp image.